Hello; I made that comment and I do agree with your speculation except for the no blame part. To me if you make a deal then you try to keep your end of it. I get that some heirs might only want the property and/or money it represents. But if they were to accept the property with it understood there are strings attached then killing off the fish is a wrong thing.
Point of clarity. I do not know the heirs at all. The OP does and has a notion of their character.
My situation is pretty straight forward. I do not have anyone at all to count on if I still have tanks set up when I pass. It could take days or even weeks before anyone found me if I suddenly passed at home. Especially now with the virus. My tanks are set up with very low stocking density and could go a long time without attention so someone might get to them in time. If I wound up in the hospital or nursing home then there might be a chance for me to at least try to get the fish to a new home.
When referring to not blaming anyone, I meant that everyone would have so much to gain by this nefarious act that picking just one to blame would be difficult, since all would have motive. These people would not have "made a deal" with anyone; rather, they would have been dictated terms by a person arrogant enough to decide that he/she insisted on continuing to control people and events from beyond the grave.
I've been an executor; I know first-hand the stresses and pressures created within a family that has already suffered a loss, and then receives another body-blow when something like this scenario occurs. Tensions build to way beyond the breaking point; arguments and disagreements that may have germinated in years or decades past are dragged up and sprout anew; the executor is somehow "to blame" for everything. Forcing one's heirs into this unpleasant situation is not being fair to them, it's just kicking them when they are already at their lowest point.
In the case of the OP, the heirs already have the estate to divide among themselves, and it will be a literal miracle if all of them agree to maintaining the
status quo. It's much more likely that at least one or more will want a cash buyout, quite likely forcing the sale unless the others can afford to pay. There will be a lot of arguing and unpleasantness even without the added and...let's be honest...ridiculous complication of a fish being the primary beneficiary.
Is the suggestion being made that the estate pay for the ongoing services of a veterinarian to watch over the interests of the fish, making sure that it is kept happy and healthy and suffers no attempts on its life? This will make a great sitcom-noir; I'd watch all three episodes that manage to be aired before the show is cancelled. I vote for Eddie Murphy to play the vet. No, wait...Eddie Murphy can be the next-door neighbour who is roped into the role of executor...the vet could be played by Will Farrell...
If the OP sets up everything ahead of time with Ohio Fish Rescue (for example...) then everyone's interests, including those of the fish, are being best served and in the most sensitive and thoughtful manner, with no additional stress to anyone.